1. Technical Field
This disclosure relates to an image forming apparatus, and more specifically to an image forming apparatus including a recording head for ejecting liquid droplets.
2. Description of the Background Art
Image forming apparatuses are used as printers, facsimile machines, copiers, plotters, or multi-functional devices having two or more of the foregoing capabilities. As one type of image forming apparatus employing a liquid-ejection recording method, an inkjet recording apparatus is known that uses a recording head formed with a liquid ejection head (liquid-droplet ejection head) for ejecting droplets of ink. During image formation, the image forming apparatuses eject droplets of ink or other liquid from the recording head onto a recording medium to form a desired image. Such inkjet-type image forming apparatuses fall into two main types: a serial-type image forming apparatus that forms an image by ejecting droplets from the recording head while moving the recording head in a main scanning direction of the carriage, and a line-head-type image forming apparatus that forms an image by ejecting droplets from a linear-shaped recording head held stationary in the image forming apparatus.
As for the recording heads (droplet ejection heads) used in these inkjet-type image forming apparatuses, several different types are known. One example is a piezoelectric recording head that ejects droplets by deforming a diaphragm using, e.g., a piezoelectric actuator. Specifically, when the piezoelectric actuator deforms the diaphragm, the volume of a chamber containing the liquid is changed. As a result, the internal pressure of the chamber increases, thus ejecting droplets from the head. Another example is a thermal recording head that ejects droplets by increasing the internal pressure of the chamber using a heater. This increase is accomplished, for example, by using a heater located in the chamber that is heated by an electric current to generate bubbles in the chamber. As a result, the internal pressure of the chamber increases, thus ejecting droplets from the head.
For such a liquid-ejection type image forming apparatus, there is demand for enhancing throughput, i.e., speed of image formation. One way to achieve enhanced throughput is to enhance the efficiency of liquid supply. For example, a liquid supply method is proposed in which ink is supplied from a high-capacity ink cartridge (main tank) mounted in the image forming apparatus to a head tank (also referred to as a sub tank or buffer tank) mounted in an upper portion of the recording head through a tube. Such a method of supplying liquid through a tube (referred to as a tube supply method or off-carriage method) can reduce not only the weight and size of a carriage with the recording head but also the entire size of the image forming apparatus including the structural and drive systems.
However, in the off-carriage method, because ink ejected from the recording head during image formation is supplied from the main tank to the recording head through the tube, for example, use of a narrow tube as a supply passage, ejection of an increased amount of ink from the recording head in high-speed printing, or use of a highly viscous ink increases resistance (referred to as fluid resistance) against ink passing through the tube. As a result, ink may not be timely supplied to the recording head, thus causing ejection failure.
Hence, for example, JP-3606282-B proposes a liquid ejection apparatus that has a valve unit disposed between the main tank and the recording head to maintain the liquid in the main tank in a pressurized state and supply the liquid to the head. The valve unit includes a pressure chamber connected to the main tank through the supply passage, a valve (also referred to as pressure-difference regulation valve or negative-pressure conjunction valve) to open and close the supply passage to supply liquid to the pressure chamber, an urging member to urge the valve in a direction to close the supply passage, and a flexible film member to deform in accordance with negative pressure created by a reduction of liquid in the pressure chamber and transmit the deformation directly to the valve to move the valve against an urging force of the urging member. The valve is opened and closed using a force by which the flexible film member is deformed by the difference between the negative pressure of the chamber and atmospheric pressure. When the pressure within the valve unit decreases to a predetermined value, the valve is opened to automatically supply ink to the valve unit.
In the above-described configuration using the negative-pressure conjunction valve, the sealing of the negative-pressure conjunction valve is performed by the urging force of the urging member. Accordingly, if the pressure device, such as a pump, fails to operate properly and cannot stop the application of pressure, excessive pressure may be applied to the ink and the negative-pressure conjunction valve becomes unable to maintain a sealed state. As a result, pressure is applied to the ink in the head tank (the valve unit) and ink leaks from the head.